1. Field of the Invention
The present invention relates to a system for detecting underground objects by a synthetic aperture method, and more particularly, to improvement of the resolution in the system.
2. Description of the Prior Art
In FIG. 2 is shown a general form of the so-called synthetic aperture underground targets detecting system which is structured of a general form of the synthetic aperture processing in use for satellite-radar and airborne radar and, in addition thereto, means for geological correction indispensable to detection of underground targets. As examples of techniques to which a system as aforementioned is basically applied, there are known such as disclosed in a paper entitled "Electromagnetic Detection of Underground Objects", pages 308-311, Proceedings of the Institute of Electronics and Communication Engineers of Japan, Vol. 67, No. 3, March 1984. In the explanatory chart of the general form in FIG. 2, ST1 is a step of collecting reflected wave profile data, ST2 is a step of performing preprocessing in succession to step ST1, ST3 is a step of performing synthetic aperture processing in succession to step ST2, ST4 is a step of performing geological correction in succession to step ST3, and ST5 is a step of performing output processing in succession to step ST4.
FIG. 3 is a diagram for explaining the collection of the reflected wave profile data at the aforesaid step ST1, referring to which reference numeral 1 denotes a target such as a pipe, 2 denotes soil in which the target 1 is buried, 3 denotes a transmitter, 4 denotes a transmitting antenna for emitting a pulse signal from the transmitter 3 as an electromagnetic wave into the aforesaid soil 2, 5 denotes a receiving antenna for receiving the wave reflected by the target 1 of the electromagnetic wave emitted from the transmitting antenna 4, and 6 denotes a receiver connected to the receiving antenna 5, where the transmitting antenna 4 and the receiving antenna 5 are fixedly held at a predetermined mutual distance y and adapted to move in increments of a predetermined distance in a direction at right angles with the direction in which both the antennas 4, 5 are disposed, as indicated by the arrow X.
Below will be described the operations. First, at step ST1, collection of reflected wave profile data is performed on a plane cutting through the soil at right angles with the ground. That is, a monocyclic pulse, for example, is emitted from the transmitting antenna 4 at every increment in the movement and the reflected wave is received by the receiving antenna 5. The reflected wave from the target 1 is received in the shortest period of time when both the transmitting antenna 4 and the receiving antenna 5 are directly above the target 1 and the time becomes longer as the antennas separate from the position right above the target, and thus parabolically spreading reflected wave profile data are obtained for each of the targets 1.
Since the monocyclic pulse propagating through the soil 2 is greatly attenuated, largely distorted, and accompanied by noises at high level, and further, since there is a direct coupling between the transmitting antenna 4 and the receiving antenna 5, the direct coupling and noises, and further, the distortions are removed in the preprocessing at step ST2. At the following step ST3, such preprocessed reflected profile data are subjected to synthetic aperture processing. That is, the reflected wave profile data are provided with range migration correction and the hyperbolic data corresponding to each target 1 are made to cohere around the vertex portion, and thereby, image data thereof are obtained.
The thus obtained image data are still those expressed with respect to the scale of time. Therefore, geological correction is performed in the following step ST4 with dielectric constant .epsilon. S of the soil 2 used, and thereby, image data expressed with respect to the scale of length are obtained, and then, the image output of the detected targets is displayed on a display device or the like at step ST5.
Since the prior art underground target detecting systems have been constructed as above, there have been such problems with them that the target spots on the obtained image data have not been sharp ones and have been affected by noises, distortions, or the like that are not completely removed in the preprocessing and therefore sufficient resolution has not been obtained.